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Originally, it was supposed to be an alternate RIFTS; Earth...one where the conflict that created the Earth we all know and love happened a couple of days early, therefore NOT creating our favourite world. Supposedly many of the nations of the world were to survive, although badly damaged.

What it came down to was it became the precursor of RIFTS: Earth.

Chaos Earth is as the others have described it. I would say yes, it could be added to your Rifts' game in a precurssor context.
I use it to segue my HUII game into Rifts.

I'd say the big element that makes it stand out storywise would be this:
In BtS, the supernatural is there, but most don't see it or believe in it.
In Rifts' the supernatural is all around us, and life has been reduced to day to day struggles for survival against countless menaces bent on enslaving or preying on humanity on a wide scale.
In Chaos Earth, things are going to Hell in a wheelbarrow, but you pick up and try to bring order to the chaos. Be brave, save lives, be a hero. Larger than life heroism is probably almost as important to this setting as it is in HUII.